1. Field of the Invention
The present invention relates generally to a drain mechanism for a hydraulic power transmission joint for use in 4 wheel-drive mechanisms for motor vehicles, and more particularly to a drain mechanism for a hydraulic power transmission joint, serving both as a temperature-based 2 wheel-drive switching mechanism and a torque limiter mechanism.
2. Description of the Related Arts
Such a hydraulic power transmission joint for use in an automobile 4 wheel-drive mechanism is hitherto known from U.S. Pat. Nos. 5,706,658 and 5,983,635.
FIG. 1A illustrates an exemplary drain mechanism for use with a hydraulic power transmission joint being currently developed by the present inventors on the basis of such U.S. Patents. A rotor is provided in the hydraulic power transmission joint for transmitting torque as a function of the rotational-speed difference between an input shaft and an output shaft, the rotor being provided with a valve block 101 that rotates jointly with the rotor. The rotor is linked to a main shaft coupled to rear wheels, for integral rotation with the main shaft. The valve block 101 is formed with an accommodation hole 102 that accommodates a drain plug 105 having a drain hole 104 through which hydraulic pressure from a high-pressure chamber 103 is discharged. Within the drain plug 105 there is slidably provided a drain pin 106 for opening and closing the drain hole 104. A fixed pin 107 is fixedly inserted into the valve block 101 for restricting the displacement of the drain pin 106. A low-pressure chamber 108 is formed in the accommodation hole 102 of the valve block 101. Within the low-pressure chamber 108 is provided a thermo-switch 111 that is urged by return springs 109 and 110 to press the drain pin 106 so as to block the drain hole 104. The thermo-switch 111 is provided with a head pin 112 that protrudes outward when a predetermined temperature is reached. In the status where the predetermined temperature is not reached for operation, the thermo-switch presses the drain pin 106 leftward with the aid of the return springs 109 and 110 as shown in FIG. 1A, to block the drain hole 104. At that time, the head pin 112 of the thermo-switch 111 does not abut against the fixed pin 107 with a minute gap therebetween. When the predetermined temperature is reached, the head pin 112 of the thermo-switch 111 is extended leftward as shown in FIG. 1B to abut against the fixed pin 107, with the resultant reaction force causing the thermo-switch 111 to retreat rightward against the biasing forces of the return springs 109 and 110. When the thermo-switch 111 retreats rightward, the force pressing the drain pin 106 is cut off, to release the drain pin 106 which in turn is displaced rightward by a high pressure from the high-pressure chamber 103 to open the drain hole 104. As a result of this, oil within the high-pressure chamber 103 flows through the drain hole 104 and via a drain chamber 114 into a drain passage 115 as indicated by the arrow.
That is, when the drain pin 106 retreats to open the drain hole 104 as shown in FIG. 2B from the status of FIG. 2A in which the drain hole 104 is blocked by the drain pin 106, oil within the high-pressure chamber 103 flows through the drain hole 104 into the drain chamber 114, and thence is drained through the drain passage 115 into the low-pressure chamber as indicated by the arrow. A further displacement of the drain pin 106 is prevented by the abutment of the bottom of a recessed portion 113 of the drain pin 106 against the fixed pin 107 as shown in FIG. 2B.
FIG. 3 illustrates torque characteristics in which characteristic A is obtained when the temperature of oil is below a predetermined level prior to the operation of the drain pin 106, with the acquisition of the torque xcex94T being proportional to the square of the rotational-speed difference. In contrast to this, characteristic B is a torque characteristic obtained after the predetermined temperature has been exceeded for the operation of the drain pin 106, in which the torque xcex94T continues to increase accordingly as the rotational-speed difference xcex94N increase since the drain capability runs short due to the smaller diameter of the drain hole 104.
However, in such a conventional drain mechanism for the hydraulic power transmission joint, the drain hole had a reduced diameter to form an orifice, which allows the hydraulic pressure to be set to a higher level but which is apt to induce a shortage of the drain capability even after the operation of the drain pin at the temperature exceeding the predetermined level, whereupon the torque still continues to increase with the increase of the rotational-speed difference. Furthermore, in case of the function as the torque limiter for keeping the torque at a predetermined level after the torque level has been reached, the torque also rose with the differential rotations due to the shortage of the drain capability.
The present invention provides a drain mechanism for a hydraulic power transmission joint, capable of alleviating insufficient drain capability and of keeping a certain torque when a predetermined torque is reached, as well as serving on the basis of the temperature as a 2 wheel-drive mechanism and a torque limiter.
The present invention is directed to a drain mechanism for a hydraulic power transmission joint adapted to be interposed between an input shaft and an output shaft that are rotatable relative to each other, to transmit torque as a function of the rotational-speed difference between the input and output shafts. According to a first aspect of the present invention, the drain mechanism comprises a limiter plug accommodated within a valve block and having a communication hole through which oil is introduced from a first high-pressure chamber; a thermo-switch located within a low-pressure chamber in the valve block in such a manner as to be urged by a return spring, the thermo-switch having a head pin that protrudes when a predetermined temperature is reached; a fixed pin fixedly inserted into the valve block, the fixed pin being positioned so as to allow the head pin of the thermo-switch urged by the return spring to abut thereagainst, the fixed pin when the head pin protrudes at the predetermined temperature causing the thermo-switch to retreat against the return spring; a limiter pin slidably located within the limiter plug, the limiter pin being pressed by the thermo-switch to close the communication hole for the duration in which the predetermined temperature is not reached, the limiter pin when the predetermined temperature is reached, opening the communication hole as a result of cutoff of the pressing force induced by a retreat of the thermo-switch, the limiter pin when a predetermined torque is reached, opening the communication hole by a high pressure from the first high-pressure chamber; a drain plug that follows the limiter plug, accommodated in the valve block, the drain plug having a drain hole through which oil is discharged from a second high-pressure chamber; and a drain pin slidably located within the drain plug in such a manner as to be urged by a drain spring, the drain pin having the first high-pressure chamber into which oil is introduced from the second high-pressure chamber and which opens to the communication hole, the drain pin closing the drain hole by means of a pressing force of the limiter pin until a predetermined temperature is reached, the drain pin rapidly opening the drain hole in response to a retreat of the limiter pin effected when the predetermined temperature is reached, the drain pin when a predetermined torque is reached for the duration in which the predetermined temperature is not yet reached, opening the drain hole while adjusting its balance so as to keep the predetermined torque.
Herein, the drain pin includes an orifice by way of which the first high-pressure chamber located in the interior of the drain pin can communicate with the low pressure side located in the exterior thereof. Furthermore, it functions also as a torque limiter by reducing the difference between the pressure receiving area associated with the second high-pressure chamber of the drain pin and the pressure receiving area associated with the first high-pressure chamber in the status where a predetermined temperature is not yet reached and the thermo-switch is out of action. The ratio of area of the pressure receiving area associated with the first high-pressure chamber of the drain pin to the pressure receiving area associated with the second high-pressure chamber is 1.1 or less for example.
According to a second aspect of the present invention there is provided a drain mechanism as described above, wherein the hydraulic power transmission joint comprises a housing coupled to the input shaft and having a cam face formed on its inner side surface; a rotor coupled to the output shaft and being rotatably accommodated in the housing, the rotor having a plurality of axially extending plunger chambers; a plurality of plungers each being reciprocatively accommodated in each of the plurality of plunger chambers under a biasing force of the return spring, the plurality of plungers being operated by the cam face upon the relative rotations of the input and output shafts; and a discharge hole formed in the rotor and opening to the plurality of plunger chambers; wherein the valve block is coupled to the rotor so that oil discharged by the operation of the plungers induces a reaction force in the plungers due to a flow resistance upon the passage through an orifice provided in the valve block, to thereby transmit torque between the housing and the rotor.
According to the thus constructed drain mechanism of the present invention, it is prevented that even though the predetermined temperature has been reached, the torque continues to increase accordingly as the rotational-speed difference increases, due to the insufficient drain capability, to thereby securely eliminate the torque. Furthermore, in the status where the predetermined temperature is not yet reached, the drain pin itself can operate, on the basis of the hydraulic balance, in response to the rise of torque, thereby enabling the torque to be kept at a certain level. This allows the temperature-rise based 4 wheel-drive to 2 wheel-drive mechanism and the torque limiter mechanism to be implemented together. As a result, it is possible to prevent any abnormal rise in temperature of the joint and to reduce the weight of the entire power train due to its ability to cut off the peak torque upon abrupt start or the like. In addition, by virtue of the formation of the orifice within the drain pin to allow the second high-pressure chamber and the first high-pressure chamber at the low pressure side to communicate with each other, the construction of the orifice mechanism can be simplified.
The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.